R1:3d3d3b6d8005

Move determination of reviewer authority into DifferentialRevisionQuery

Summary:
Ref T1279. We currently determine reviewers at display time, but this is bad for several reasons:

- It puts queries very close to the display layer.
- We have to query for each revision if we want to figure out authority for several.
- We need to figure it out in several places, so we'll end up with copies of this logic.
- The logic isn't trivial (exceptions for the viewer, exceptions to that rule for install…